Silicone water based elastomers

ABSTRACT

An improved aqueous silicone emulsion which cures to a reinforced elastomer upon removal of water is obtained through the use of an anionic, stable dispersion of fumed silica in water as the source of reinforcing silica. The fumed silica dispersion is combined with an aqueous, anionic emulsion of crosslinkable polydiorganosiloxane and a crosslinking system.

This is a continuation-in-part of copending application(s) Ser. No.07/526,478 filed on May 21, 1990, now abandoned, which is a divisionalof Ser. No. 07/401,485 now U.S. Pat. No. 4,957,963.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to silicone water based elastomers reinforcedwith fumed silica filler.

2. Background Information

Aqueous silicone emulsions which yield elastomers when the water isremoved are known. A silicone emulsion having a dispersed phase of ananionically stabilized hydroxylated polydiorganosiloxane and a colloidalsilica and a continuous phase of water in which the pH is 9 to 11.5,which provides an elastomeric product upon removal of the water atambient conditions is taught in U.S. Pat. No. 4,221,688, issued Sep. 9,1980. The colloidal silica is added in the form of a dry powder or as anaqueous dispersion, preferably as a sodium ion stabilized aqueousdispersion of colloidal silica.

U.S. Pat. No. 4,427,811, issued Jan. 24, 1984, discloses a siliconeelastomeric emulsion having improved shelf life. The method ofmanufacture mixes an anionically stabilized, hydroxyl endblockedpolydiorganosiloxane, colloidal silica, and an alkyl tin salt, ages themixture for at least two weeks, then adds the filler other thancolloidal silica.

U.S. Pat. No. 4,584,341, issued Apr. 22, 1986, teaches a method ofproducing a latex of crosslinked polydiorganosiloxane which yields anelastomer upon removal of the water. The latex can be reinforced byadding greater than 1 part by weight of colloidal silica sol orsilsesquioxane.

U.S. Pat. No. 4,634,733, issued Jan. 6, 1987, discloses a method ofproducing a silicone elastomer from an emulsion. The latex can be usedimmediately upon mixing of the ingredients. The composition includescolloidal silica.

U.S. Pat. No. 4,661,556, issued Apr. 28, 1987 teaches a method ofproducing a colloidal silica reinforced polydiorganosiloxane emulsion.The method combines an acidic aqueous colloidal silica sol and lowmolecular weight hydroxyl endblocked polydiorganosiloxane with anionicemulsifying agent, then homogenizes the mixture to form an emulsion. Theemulsion is then mixed with surface active anionic catalyst andpolymerized, then neutralized. A curable emulsion can be produced bymixing the reinforced polydiorganosiloxane emulsion withdiorganotindicarboxylate and alkylorthosilicate.

U.S. Pat. No. 4,954,565, issued Sep. 4, 1990, teaches the use of tin(II) (stannous) catalyst in aqueous silicone emulsions of anionicallystabilized, hydroxyl endblocked polydiorganosiloxane, silanecrosslinker, and colloidal silica reinforcement.

Aqueous dispersions of pyrogenic silica are described in U.S. Pat. No.2,984,629, issued May 16, 1961. A pyrogenic silica is worked until theapparent bulk density is increased to between about 6 and 15 lbs/cu.ft.,mixed with water, and subjected to high shear in the presence of asulfonic acid.

SUMMARY OF THE INVENTION

An aqueous silicone emulsion which cures to an elastomer upon removal ofthe water can be reinforced by using anionic, stable dispersion of fumedsilica dispersed in water as the source of silica to give a compositionwhich yields higher physical strength than a similar composition made byreinforcing with colloidal silica.

DESCRIPTION OF THE INVENTION

This invention relates to an aqueous silicone emulsion which yields areinforced elastomer upon removal of the water, said emulsion comprisingan aqueous, anionic emulsion of crosslinked polydiorganosiloxane and aseparately prepared anionic, stable dispersion of fumed silica in water.

The aqueous silicone emulsion which yields a reinforced elastomer uponremoval of the water is the product obtained when a crosslinkablepolydiorganosiloxane, such as a hydroxy terminated polydiorganosiloxane,a multifunctional alkoxysilane such as an alkyltrialkoxysilane, and anorganic tin compound are present in the composition along with theseparately prepared anionic, stable dispersion of fumed silica in water.The organic tin compound is preferably an organic stannous compound,such as stannous octoate. When such an emulsion is formed, the hydroxyterminated polydiorganosiloxane, alkyltrialkoxysilane, and organic tincompound are all present in the dispersed phase micelle and they reacttogether to give crosslinking of the polymer. The degree of crosslinkingnecessary is that which gives an elastomer upon removal of the waterfrom the dispersion.

Dispersions of anionic, stabilized fumed silica in water arecommercially available at Ph varying from about 5 to 11. The amount offumed silica in the emulsion varies from about 10 to 30 percent byweight. Generally, as the solids content of the emulsion is raised, theparticle size of the silica is larger. Dispersions of fumed silica inwater are offered by Cab-O-Sil Division of Cabot Corporation under thetrademark "CAB-O-SPERSE". The dispersions are stated to be stabilizedwith ammonia and with potassium hydroxide. The CAB-O-SIL fumed silicaused in CAB-O-SPERSE dispersions is a synthetic amorphous silicaproduced by the hydrolysis of silicon tetrachloride vapor in a flame ofhydrogen and oxygen. This flame hydrolysis method of silica manufactureresults in a product characterized by an unusual combination of physicaland chemical properties. The physical properties include high externalsurface area, low density, and a three-dimensional chain-like structureof the primary particles. These properties of the fumed silica areretained in the dispersion of the silica in water which is used in thisinvention. The nominal particle sizes in various commercially availableCAB-O-SPERSE grades vary from 7 to 30 nanometers. Preferably, ananionic, stabilized fumed silica dispersed in water having a pH above 7is used when the dispersion is added to an emulsion of crosslinkablepolydiorganosiloxane. When the fumed silica dispersion is used in themethod where the silica is present during the polymerization of thepolydiorganosiloxane oligomer, it is preferred that the pH of the silicadispersion be below 7.

The above described anionic, stabilized fumed silica dispersions aredifferent from the aqueous sodium ion stabilized colloidal silica suchas is described in U.S. Pat. No. 4,221,688, issued Sep. 9, 1980. Thecolloidal silicas described there include solid fume colloidal silicasand precipitated colloidal silicas, the preferred colloidal silicasbeing those which are available in an aqueous medium. Those inventorsstate that colloidal silicas in an aqueous medium are usually availablein a stabilized form, such as those stabilized with sodium ion, ammonia,or an aluminum ion; and that aqueous colloidal silicas which have beenstabilized with sodium ion are particularly useful for their invention.They are referring to aqueous silica sols, generally prepared byremoving all but a small portion of the alkali metal present in waterglass to produce a colloidal system of what might be described aspolymeric silicic acid. One process for taking such a product andconcentrating it to a useful concentration is described in U.S. PatentReissue No. Re 25,252, reissued Oct. 9, 1962, to Nalco Chemical Company.Commercial products are available such as Nalcoag 1115, a 15 percentsolids colloidal silica dispersion having silica particles of a surfacearea of about 750 m² /g, Nalcoag 1030, a 30 percent solids colloidalsilica dispersion having silica particles of a surface area of about 375m² /g, and Nalcoag 1050, a 50 percent solids colloidal silica dispersionhaving silica particles of a surface area of about 150 m² /g, all fromNalco Chemical Company.

Various methods may be used to combine the ingredients necessary to givea crosslinkable polydiorganosiloxane emulsion and the anionic, stabledispersion of fumed silica in water to yield the composition of thisinvention, which is an aqueous silicone emulsion which yields areinforced elastomer upon removal of the water. The emulsion, beforeremoval of the water, comprises an aqueous, anionic emulsion ofcrosslinked polydiorganosiloxane and the fumed silica from the anionic,stable dispersion of fumed silica which is used in the preparation ofthe final emulsion.

A method of making the emulsion crosslinked polydiorganosiloxane whichyields a reinforced elastomer upon removal of the water is one made inaccordance with U.S. Pat. No. 4,221,688, is issued Sep. 9, 1980, whichpatent is hereby incorporated by reference to show a suitable emulsionand the method of its manufacture. This invention prepares a siliconeemulsion by emulsifying a hydroxylated polydiorganosiloxane whichcontains about 2 silicon-bonded hydroxyls per molecule using anionicsurfactant and water, adding a colloidal silica and an organic tincompound, and adjusting the pH of the resulting emulsion within therange from 9 to 11.5 inclusive. When this method is modified so that ananionic, stable dispersion of fumed silica is used in place of thecolloidal silica taught in the method of '688, the method becomes amethod of producing the emulsion of the instant invention. The elastomerproduced by removal of the water has a higher tensile strength when theanionic, stable dispersion of fumed silica is used to supply the silicathan when the colloidal silica called for in the '688 patent is used.The crosslinking process in this method is very slow and the emulsionneeds to age for a considerable period of time, such as 2 months orlonger, before sufficient crosslinking is present in order to give anelastomer having reasonable mechanical properties upon removal of thewater.

An aqueous, anionic emulsion of polydiorganosiloxane can be produced bythe method of U.S. Pat. No. 3,294,725, issued Dec. 27, 1966, whichteaches homogenizing an organosiloxane and polymerizing using a surfaceactive sulfonic acid catalyst. This procedure yields an aqueous, anionicemulsion of crosslinkable polydiorganosiloxane. When desired themolecular weight of the polymer can be greater than 50,000. When such anemulsion is combined with a crosslinking agent of the formula

    R.sub.n Si(OR').sub.4-n

where n is 0 or 1, R and R' are hydrogen or monovalent hydrocarbonradicals having from 1 to 6 carbon atoms, and an organic tin compound,the resulting product gives an elastomer upon removal of the water. Whenan anionic, stable dispersion of fumed silica in water is also included,the aqueous silicone emulsion of this invention results.

Examples of organic tin compound which are useful as a catalyst includedialkyltindicarboxylates, such as dibutyltindiacetate,dibutyltindilaurate, and dioctyltindilaurate; and stannous carboxylates,such as stannous oleate, stannous 2-ethylhexanoate, and stannousnaphthanate.

When the aqueous silicone emulsion is the product obtained by mixing anaqueous, anionic emulsion of crosslinkable polydiorganosiloxane,alkoxysilane crosslinker, organic tin compound, and the anionic, stabledispersion of fumed silica, the crosslinking reaction is relativelyrapid. Crosslinking to give an elastomeric product upon removal of thewater from the emulsion can take place in as little as from 30 minutesto 2 days. The amount of crosslinking can be measured experimentally becasting a film of the emulsion, removing the water, and evaluating theresultant film. The rate of crosslinking depends upon the amount andkind of both crosslinker and organic tin compound selected.

An aqueous silicone emulsion which cures to a reinforced elastomer uponremoval of the water results when 100 parts by weight of the abovedescribed aqueous, anionic emulsion of crosslinkablepolydiorganosiloxane having hydroxyl endblocking and a molecular weightof greater than 50,000 is mixed with from 1 to 40 parts by weight offumed silica anionically dispersed in water, and with from 0.2 to 2.0parts by weight of a crosslinking agent of the formula

    R.sub.n Si(OR').sub.4-n

where n is 0 or 1, R and R' are hydrogen or monovalent hydrocarbonradicals having from 1 to 6 carbon atoms, and from 0.1 to 2.0 parts byweight of an organic tin compound.

The anionic emulsion of crosslinkable polydiorganosiloxane can be mixedwith the anionic, stabilized fumed silica dispersion, the crosslinkingagent, and the organic tin compound and allowed to react. The productwill yield an elastomer upon removal of the water.

Another useful method mixes the anionic emulsion of crosslinkablepolydiorganosiloxane, anionic stabilized fumed silica dispersion, andorganic tin compound, then ages, followed by addition of thecrosslinking agent. Yet another method mixes the anionic emulsion ofcrosslinkable polydiorganosiloxane, crosslinking agent, and organic tincompound, ages, then adds the anionic, stabilized fumed silicadispersion.

The composition can also be produced by a method which polymerizes apolydiorganosiloxane in the presence of the filler by a methodcomprising (A) homogenizing (1) 100 parts by weight of hydroxylendblocked polydiorganosiloxane oligomer, (2) 1 to 40 parts of fumedsilica anionically dispersed in water, (3) optionally additional water,and (4) surfactant, then (B) adding sufficient acid to adjust the pH toless than 3, then (C) allowing the oligomer to polymerize to the desireddegree, then (D) terminating polymerization by raising the pH to greaterthan 9, and (E) adding (5) 0.1 to 2.0 parts by weight of organic tincompound, then (F) admixing (6) from 0.2 to 2.0 parts by weight ofcrosslinking agent of the formula

    R.sub.n Si(OR').sub.4-n

where n is 0 or 1, R and R' are hydrogen or monovalent hydrocarbonradicals having from 1 to 6 carbon atoms, to yield an aqueous siliconeemulsion which cures to a reinforced elastomer upon removal of thewater.

The organic radicals of the polydiorganosiloxane oligomer can bemonovalent hydrocarbon radicals containing less than seven carbon atomsper radical and 2-(perfluoroalkyl)ethyl radicals containing less thanseven carbon atoms per radical. The oligomer preferably contains organicradicals in which at least 50 percent are methyl. The surfactant ischosen so that it forms a stable emulsion and also allows the oligomerto be polymerized. A preferred surfactant is an alkyl sulfate, thepreferred alkyl sulfate is sodium lauryl sulfate.

When sodium lauryl sulfate is used as the surfactant (4), the acid ofstep (B) is preferably a dilute mineral acid, such as hydrochloric acid.The dilute hydrochloric acid reacts with the sodium lauryl sulfate togive hydrogen lauryl sulfate, which is an active polymerization catalystfor the polydiorganosiloxane oligomer.

The surfactant of (4) can also be a surface active sulfonic acid suchare described in U.S. Pat. No. 3,294,725, issued Dec. 27, 1966, toFindley and Weyenberg, which patent is hereby incorporated by referenceto show suitable surfactants and polymerization catalysts for theemulsion polymerization of polydiorganosiloxane oligomer. When a surfaceactive sulfonic acid such as a preferred dodecylbenzene sulfonic acid isused as surfactant (4), the pH of the mixture will be less than 3, so itis no longer necessary to add another acid as is called for in step (B)of this method.

In either case, the oligomer is then allowed to polymerize to thedesired degree. This polymerization will take place over time at roomtemperature, one or two days is generally sufficient to yield a highmolecular weight polymer. The polymerization rate can be raised byraising the temperature.

After the polymerization has proceeded to the desired degree, thepolymerization is terminated by raising the pH of the emulsion togreater than 9. The preferred method is the addition of dilute aqueoussodium hydroxide or an organic amine compound such as diethylamine or2-amino-2-methyl-1-propanol. The preferred method uses a 20 percentaqueous diethylamine dispersion.

The emulsion is then catalyzed and made curable by the addition ofcatalyst and crosslinker. The catalyst is an organic tin compound inwhich the alkyl radical is a radical such as butyl or octyl, and thecarboxylate is a radical such as acetate, laurate, or octoate. Thepreferred catalyst is dioctyltindilaurate or stannous octoate (stannous2-ethylhexanoate). The amount of catalyst is not critical and can varyfrom 0.1 to 2.0 parts by weight per 100 parts by weight of oligomer. Thepreferred amount is about 0.5 parts, which gives a crosslinked polymerin the emulsion in about 1 day. The time required for the polymer tobecome crosslinked varies with the amount of catalyst used and thecrosslinker chosen.

The crosslinker is of the formula R_(n) Si(OR')_(4-n) where n is 0 or 1,R and R' are hydrogen or monovalent hydrocarbon radicals having from 1to 6 carbon atoms. Useful crosslinking agents includevinyltrimethoxysilane, methyltrimethoxysilane, ethylorthosilicate, andmethylorthosilicate, with the preferred crosslinking agent beingvinyltrimethoxysilane.

The above method allows the production of an emulsion having a highersolids content in that the water present in ingredient (2) can be partor all of the water used to make the emulsion.

The methods described above produce an improved emulsion which cures toan elastomer upon removal of the water, the elastomer having improvedphysical properties because of the anionic, stable dispersion of fumedsilica that was used to produce the emulsion. The emulsion is useful asa coating material. It can be further modified with additional dispersedfumed silica, extending fillers, pigments, and such to yield other typesof coatings and sealants.

The following examples are included for illustrative purposes only andshould not be construed as limiting the invention which is properly setforth in the appended claims. All parts are parts by weight.

EXAMPLE 1

Fifty grams of an emulsion of an anionic, emulsion polymerizedpolydimethylsiloxane having a molecular weight of about 200,000, theemulsion having a solids content of about 60 percent by weight and a pHof about 10 was placed in a flask and mixed with 27.4 g of CAB-O-SPERSE(R) SC-2 dispersed silica (pH of about 8.8, a silica surface area of 160m² /g, and 18 percent solids) to give 15 parts of silica per 100 partsof polymer. To the stirring mixture was added 0.62 g of 50 percentsolids by weight emulsion of dioctyltindilaurate and 0.16 g ofvinyltrimethoxysilane. The mixture was then allowed to react at roomtemperature for 48 hours. A sample of this material was poured into apetri dish and allowed to dry. After 8 more days another sample waspoured into a petri dish and allowed to dry. At 4 weeks after mixing,the dried samples were measured for physical properties with theseresults:

    ______________________________________                                        Emulsion age  Tensile Strength                                                                           Elongation                                         after mixing  MPa          percent                                            ______________________________________                                         2 days       2.34         855                                                10 days       3.18         900                                                ______________________________________                                    

EXAMPLE 2

A sample was prepared as in Example 1 by mixing the polydimethylsiloxaneemulsion, the silica emulsion, and the dioctyltindilaurate emulsion.This mixture was allowed to age at room temperature for 48 hours. Then0.16 g of vinyltrimethoxysilane was added. Periodically samples werepoured into petri dishes and allowed to dry for the times as shownbelow. One month after mixing, the dried samples were tested forphysical properties with the following results:

    ______________________________________                                        Emulsion age                                                                  after addition of                                                                            Tensile Strength                                                                           Elongation                                        vinyltrimethylsilane                                                                         Mpa          percent                                           ______________________________________                                        10 minutes     2.16         708                                                2 days        2.54         768                                               10 days        2.60         775                                               ______________________________________                                    

A similar emulsion reinforced by dispersing colloidal silica in theemulsion in the same amount typically has about 1.7 MPa tensile strengthand about 400 percent elongation.

EXAMPLE 3

A 600 g portion of the polydimethylsiloxane emulsion of Example 1 wascrosslinked by mixing it with 1.86 g of vinyltrimethoxysilane and 1.49 gof the catalyst emulsion of Example 1, then adjusting to a pH of about9.8 by adding diethylamine. The emulsion was then stored in a closedcontainer for 20 days. After the storage period, 20 g samples wereplaced in vials, then a sample of aqueous, dispersed fume silica wasadded to each vial as shown in the following table. The silicadispersions were added so that each emulsion contained 15 parts or 20parts of silica per 100 parts of polymer. Each vial was shaken for 10minutes to mix, then centrifuged to exclude air bubbles. Each sample wasthen poured into a petri dish and allowed to air dry at ambientconditions. After air drying for 6 days, the films were measured forphysical properties with the results shown in the following Table I.

                                      TABLE IV                                    __________________________________________________________________________    Silica Level                                                                         Silica Surface Area                                                                     percent  pH  Tensile Strength                                                                       Elongation                             pph    m.sup.2 /g                                                                              Solids                                                                            Viscosity                                                                          cps MPa      percent                                __________________________________________________________________________    15      90       28  58    9.76                                                                             1.17     730                                    15     130       25  --   10.3                                                                              2.14     960                                    15     200       17  24    9.9                                                                              2.07     700                                    15     250       16  62   10.0                                                                              2.07     740                                    15     380       14  <100 10.0                                                                              1.86     580                                    20      90       28  58    9.76                                                                             2.00     1200                                   20     130       25  --   10.3                                                                              2.20     1200                                   20     200       17  24    9.9                                                                              1.86     1200                                   20     250       16  62   10.0                                                                              1.79     1000                                   20     380       14  <100 10.0                                                                              1.52     1040                                   __________________________________________________________________________

EXAMPLE 4

A polydiorganosiloxane was prepared in the presence of filler by mixing200 g of hydroxyl endblocked polydimethylsiloxane oligomer (molecularweight about 2600) with 100 g of CAB-O-SPERSE (R) B, a 15 percent solidsdispersion of fumed silica having a surface area of about 200 m² /g anda pH of about 5.3, 130 g of deionized water, and 21 g ofsodiumlaurylsulfate surfactant to give 5 parts of silica per 100 partsof polymer. This mixture was homogenized at 6000 psi in a laboratoryhomogenizer by giving three passes through the device to obtain a stableemulsion. The pH of the emulsion was lowered to 1.5 by admixingdodecylbenzenesulfonic acid. The mixture was allowed to polymerize for 5days at room temperature, after which the emulsion was neutralized withdiethylamine.

Then 26 g of the above emulsion was mixed with 7.94 g of a silicadispersion having 17 percent solids and 0.279 g of the catalyst ofExample 1. This gave a total silica content of 15 parts per 100 parts ofpolymer. After 3 days at room temperature, 0.135 g ofvinyltrimethoxysilane was added and samples were poured into petridishes and allowed to dry. The dried film had a tensile strength of 3.48MPa and an elongation of 890 percent.

EXAMPLE 5

An emulsion was prepared using the anionic, stable dispersion of fumedsilica in water used in this invention and a comparative emulsion wasprepared using colloidal silica to illustrate the difference in theproducts produced by the two different compositions.

First, an Emulsion A was prepared by weighing into a jar 250 g of ananionically stabilized hydroxy terminated polydimethylsiloxane emulsionhaving a solids content of about 70 percent by weight and a pH of about10, the emulsion particles being less than 0.6 micrometers in averagediameter and the polymer having a molecular weight average of greaterthan 240,000. Then the pH of the emulsion was raised to approximately 11by addition (with stirring) of 2-amino-2-methyl-1-propanol (AMP-95).Next, 1.75 g of 50 percent solids by weight emulsion ofdioctyltindilaurate was added and the emulsion was stirred for severalminutes. Next, 145.8 g of CAB-O-SPERSE SC-2 aqueous, dispersed, fumedsilica was added, the jar was closed, the mixture was shaken for severalminutes and allowed to stand for three days at room temperature withoutagitation. Next, 0.88 g of vinyltrimethoxysilane (VTM) was added to theemulsion with good stirring. The jar was recapped and the mixture wasallowed to stand for two days at room temperature without agitation.Approximately 10 g of this mixture was placed in a vial, centrifugedlightly to exclude air bubbles, poured into a 100 mm diameter petri dishand allowed to dry at ambient conditions for one week. Mechanicalproperties of the resulting film were determined, the results are shownin Table II. Emulsion A contained 0.5 parts tin catalyst, 0.5 part VTMand 15 part SC-2 silica, all based on 100 parts of polymer.

An Emulsion B was prepared at the same time as Emulsion A and using thesame procedure, except 52.5 g of Nalco 1050 colloidal silica was used inplace of the SC-2 silica. Mechanical properties of the elastomeric filmfrom Emulsion B were also determined at the same time as those of thefilm from emulsion A, as shown in Table II. Emulsion B contained 0.5part tin catalyst, 0.5 part VTM, and 15 parts 1050 colloidal silica, allbased on 100 parts of polymer.

                                      TABLE II                                    __________________________________________________________________________          Type SiO2  Surface area                                                                         Tensile Strength                                                                       Elongation                                   Elastomer                                                                           (from)                                                                              Amount                                                                             m2/g   (MPa)    (%)                                          __________________________________________________________________________    emulsion A                                                                          SC-2  15 pph                                                                             160    2.41     960                                                (Cabot)                                                                 emulsion B                                                                          1050  15 pph                                                                             150    0.69     680                                                (Nalco)                                                                 __________________________________________________________________________

A comparison of these results clearly shows the superiority of the useof the anionic, stable dispersion of fumed silica in water as the sourceof silica.

EXAMPLE 6

An emulsion 1 was prepared by placing into a one pint jar equipped witha mechanical stirrer 250 g of the anionically stabilizedpolydimethylsiloxane emulsion of Example 5. The stirrer was started and0.88 g of vinyltrimethoxysilane was added dropwise to the emulsion,followed by 1.75 g of a 50 percent solids emulsion ofdioctyltindilaurate. Next, the pH of the emulsion was raised toapproximately 11 by addition (with stirring) of 3 g of2-amino-2-methyl-1-propanol (AMP-95). Next, 146 g of CAB-O-SPERSE SC-2,aqueous, dispersed, fumed silica was poured into the emulsion withstirring. Stirring was continued for about 5 minutes, after which thejar was closed and the mixture was allowed to stand for three days atroom temperature without agitation. This emulsion contained 0.5 pphvinyltrimethoxysilane, 0.5 pph tin catalyst, and 15 pph silica, allbased on polymer by weight.

After three days, the jar was shaken to mix the ingredients and a 30 galiquot was poured into a vial. The vial was lightly centrifuged toexclude air bubbles and approximately 8 g of this mixture was pouredinto a 100 mm diameter petri dish and allowed to dry under ambientconditions. After 5 days, mechanical properties of the resultingelastomeric film were determined; the results are shown in table III.

An Emulsion 2 was produced in the same manner as emulsion 1 except thatfumed silica was substitued for the aqueous, dispersed, fumed silica.The same procedure for preparing emulsion 1 was used to prepare emulsion2 up to the point of addition of silica. For emulsion 2, 120 g ofdeionized water was added to the emulsion with stirring followed by 26.3g of Cabot (R) LM-7 fumed silica. Fumed silica was added to the emulsionin approximately 2-3 g increments spaced about 1-2 minutes apart so thatthe silica could become dispersed throughout the emulsion. When the lastone quarter of silica was yet to be added, the mixture had a consistencyof thick paste and mixing became difficult. An additional 5 g ofdeionized water had to be added to the mixture in order that theremainder of silica could be added and properly dispersed into theemulsion. After all of the silica was added, the mixture was stirred foran additional 15 minutes, the jar was capped and the mixture was allowedto stand for 3 days at room temperature without agitation.

Using the same procedures as those used for emulsion 1, a film was castfrom emulsion 2 and its mechanical properties were determined; theresults are given in table III.

Emulsion 2 contained 0.5 pph vinyltrimethoxysilane, 0.5 pph tincatalyst, and 15 pph Silica (by weight based on polymer); it was thesame as emulsion 1 except that it contained LM-7 fumed silica in placeof SC-2 aqueous, dispersed, fumed silica. Emulsion 2 also containedslightly more water than did emulsion 1. Since SC-2 aqueous, dispersed,fumed silica is LM-7 fumed silica dispersed in water, the onlysignificant difference between emulsions 1 and 2 is the manner in whichsilica was incorporated into each mixture.

                  TABLE III                                                       ______________________________________                                                         Amount                                                       Emulsion                                                                             Type Silica                                                                             Silica   Tensile Strength                                                                         Elongation                               ______________________________________                                        1      SC-2      15 pph   2.51 MPa   1300%                                    2      LM-7      15 pph   1.10 MPa    815%                                    ______________________________________                                    

To 200 g of an anionically emulsified polydimethylsiloxane emulsionhaving a solids content of about 70 percent by weight and a pH of 10.5,the emulsion particles being less than 0.6 micrometers in averagediameter and the polymer having a molecular weight average of greaterthan 240,000, in a 1 pint jar was added 0.7 g of stannous octoate. Theemulsion was stirred for one minute to disperse the tin catalyst andstirring was continued while 1.4 g of3,3,3-trifluoropropyl(methyl)trimethoxysilane was added dropwise.

After all of the alkoxysilane crosslinker had been added, the emulsionwas stirred for an additional two minutes. The jar was capped and theemulsion was allowed to remain undistrubed for one hour. Six 20 galiquot of the crosslinked emulsion were weighed into vials. Next,sufficient aqueous, dispersed, fumed silica having a pH of 7.0, a solidscontent of 12 percent by weight and a surface area of 380 m2/g(CAB-O-SPERSE (R) F) was added to five of the vials such that the sixsamples now formed a series having the following levels of fumed silica:0, 5.0, 7.5, 10.0, 12.0, and 15.0 parts fumed silica (solid) per 100parts of silicone polymer (dry). Each vial was shaken to disperse theingredients followed by a light centrifugation to exclude air bubbles.

Films were cast by pouring approximately 8 g of emulsion into a 100 mmdiameter Petri dish and allowing the water to evaporate. Seven daysafter the films had been cast, mechanical properties were determined andthey are given in Table IV.

                  TABLE IV                                                        ______________________________________                                        SiO2 Level      Tensile Strength                                                                           Elongation                                       pph based on polymer                                                                          MPa          %                                                ______________________________________                                        0               0.21          450                                              5.0            0.76         1300                                              7.5            1.52         1440                                             10.0            2.55         1280                                             12.0            1.93          970                                             15.0            2.20          810                                             ______________________________________                                    

That which is claimed is:
 1. An aqueous silicone emulsion which yields areinforced elastomer upon removal of the water, said emulsion comprisingan aqueous, anionic emulsion of crosslinked polydiorganosiloxane and aseparately prepared anionic, stable dispersion of fumed silica in water.2. An aqueous silicone emulsion which yields a reinforced elastomer uponremoval of the water, said emulsion comprising an aqueous, anionicemulsion of crosslinked polydiorganosiloxane and fumed silica, thecrosslinked polydiorganosiloxane being the product obtained by mixing acrosslinkable polydiorganosiloxane and a suitable crosslinking system,and the fumed silica being derived from a separately prepared anionic,stable dispersion of fumed silica in water.
 3. The aqueous siliconeemulsion of claim 2 in which the crosslinkable polydiorganosiloxane ishydroxy terminated and there is also present a multifunctionalalkoxysilane and an organic tin compound in amounts sufficient tocrosslink the polydiorganosiloxane.
 4. The aqueous silicone emulsion ofclaim 3 in which the multifunctional alkoxysilane is analkyltrialkoxysilane and the organic tin compound is an organic stannouscompound.
 5. The aqueous silicone emulsion of claim 4 in which thealkyltrialkoxysilane is vinyltrimethoxysilane and the organic stannouscompound is stannous octoate.